Reforming with hydrogen, or hydroforming, is a well established industrial process employed by the petroleum industry for upgrading virgin or cracked naphthas for the production of high octane gasoline. Reforming is defined as the total effect of the molecular changes, or hydrocarbon reactions produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins to produce gas and coke, the latter being deposited on the catalyst. Historically, noble metal catalysts, notably platinum supported on alumina, have been employed for this reaction. More recently, polymetallic catalysts consisting of platinum-rhenium, platinum-iridium, platinum-tin, or various combinations thereof promoted with any one or more of the following elements copper, selenium, sulfur, chloride, and fluoride, are being utilized.
In a typical process, a series of reactors are provided with fixed beds of catalyst which receive upflow or downflow feed, and each reactor is provided with a preheater or interstage heater, because the desirable reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle gas, is cocurrently passed through a reheat furnace and reactor, and then in sequence through subsequent heaters and reactors of the series. The vapor effluent from the last reactor of the series is a gas rich in hydrogen, which usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production; coke invariably forming and depositing on the catalyst during the reaction.
Essentially all petroleum naphtha feeds contain sulfur, a well known catalyst poison which can gradually accumulate upon and poison the catalyst. Most of the sulfur, because of this adverse effect, is generally removed from feed naphthas, e.g., by hydrofining with conventional hydrodesulfurization catalysts consisting of the sulfides of cobalt or nickel and molybdenum supported on a high surface area alumina. The severity of hydrofining can be increased so that essentially all the sulfur is removed from the naphtha in the form of H.sub.2 S. However, small quantities of olefins are also produced. As a consequence, when the exit stream from the hydrofiner is cooled, sulfur can be reincorporated into the naphtha by the combination of H.sub.2 S with the olefins to produce mercaptans.
In reforming, sulfur compounds, even at a 1-20 parts per million weight range contribute to loss of catalyst activity and C.sub.5.sup.+ liquid yield. In the last decade, in particular, polymetallic metal catalysts have been employed to provide, at reforming conditions, improved catalyst activity, selectivity and stability. Thus, additional metallic components have been added to the more conventional platinum catalysts as promotors to further improve, particularly, the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, selenium, tin, and the like. In the use of these catalysts it has become essential to reduce the feed sulfur to only a few parts per million by weight, wppm. For example, in the use of platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed well below about 2 wppm, and preferably well below about 0.1 wppm, to avoid excessive loss of catalyst activity and C.sub.5.sup.+ liquid yield. By removing virtually the last traces of sulfur from the naphtha feed, catalyst activity and C.sub.5.sup.+ liquid yield of high octane gasoline can be significantly increased.
The sulfur-containing feed, prior to reforming, is hydrofined over a Group VI-B or Group VIII catalyst, e.g., a Co/Mo catalyst, and a major amount of the sulfur is removed. Residual sulfur is then generally removed from the naphtha feeds by passage through a "sulfur trap." Within the sulfur trap residual sulfur is removed from the naphtha feeds by adsorption over copper chromite, nickel, cobalt, molybdenum, and the like. These and other metals have been found useful per se, or have been supported on high surface area refractory inorganic oxide materials such as alumina, silica, silica/alumina, clays, kieselguhr, and the like. Massive nickel catalysts, or catalysts containing from about 10 percent to about 70 percent nickel, alone or in admixture with other metal components, supported on an inorganic oxide base, notably alumina, have been found particularly effective in removing sulfur from naphtha feeds, notably naphtha feeds containing from about 1 to about 50 ppm sulfur, or higher.
The sulfur trap which contains a nickel catalyst has been found to perform admirably well, both in its ability to effectively remove sulfur from the feed, and over prolonged periods of operation. Albeit the hydrofined feed usually contains from about 1 wppm to about 5 wppm sulfur, it can contain as much as 50 wppm sulfur, and higher, during periods of hydrofiner upset. The sulfur trap containing a nickel catalyst has been found suitable for removing sulfur from the hydrofined feed, lowering the sulfur to a level of 0.1 ppm, and less. The sulfur concentration, flow rate and operating temperature of the feed entering the nickel catalyst-containing sulfur trap, have been found to be critical to the quality of the product output from the sulfur trap. Preferred temperatures lie in the range of 300.degree.-500.degree. F. At temperatures below about 200.degree. F. and sulfur feed concentrations of about 50 ppm, the product from the sulfur trap is often found to contain both nickel and sulfur as an organosulfur nickel complex. The organosulfur nickel complex, which often produces a burgundy or reddish-brown color in the product from the nickel catalyst-containing sulfur trap, is detrimental to polymetallic platinum-containing reforming catalysts. In many refineries, the product from the naphtha hydrofiner has a temperature of 100.sqroot.-200.degree. F. It is economically desirable to be able to operate the nickel sulfur trap at these temperatures and thus, to eliminate the need for a heat exchanger, it becomes a necessity to provide a means for removing the soluble organosulfur nickel complex from the product.